[PATCH] sched: Buggy comparison in check_preempt_tick

[Venkatesh Pallipadi <venki@google.com>]

A preempt comparison line in check_preempt_tick has two bugs.
* It compares signed and unsigned quantities, which breaks when signed
  quantity happens to be negative
* It compares runtime and vruntime, which breaks when there are niced tasks

The bug was initially found by linsched[1]. Change here fixes both
the problems.

On x86-64, the signed unsigned compare results in tasks running _longer_
than their expected time slice as a false resched_task() gets signalled after
4 ticks (on tick after preceding sysctl_sched_min_granularity check) and
currently running task gets picked again and runs for another ideal_slice
interval.

With 2 busy loops on a single CPU and trace_printks inside this buggy check
triggering resched task and in pick_next_task shows this:

 [001]   510.524336: pick_next_task_fair: loop (5939)
 [001]   510.536326: pick_next_task_fair: loop (5883)
 [001]   510.540319: task_tick_fair: delta -4897059, ideal_runtime 11994146
 [001]   510.540321: pick_next_task_fair: loop (5883)
 [001]   510.544306: task_tick_fair: delta -906540, ideal_runtime 11994146
 [001]   510.544309: pick_next_task_fair: loop (5883)
 [001]   510.556306: pick_next_task_fair: loop (5939)
 [001]   510.560301: task_tick_fair: delta -7105824, ideal_runtime 11994146
 [001]   510.560304: pick_next_task_fair: loop (5939)
 [001]   510.564298: task_tick_fair: delta -3105461, ideal_runtime 11994146
 [001]   510.564300: pick_next_task_fair: loop (5939)
 [001]   510.576288: pick_next_task_fair: loop (5883)
 [001]   510.580282: task_tick_fair: delta -4897210, ideal_runtime 11994146
 [001]   510.580285: pick_next_task_fair: loop (5883)
 [001]   510.584278: task_tick_fair: delta -897348, ideal_runtime 11994146
 [001]   510.584281: pick_next_task_fair: loop (5883)
 [001]   510.596269: pick_next_task_fair: loop (5939)

That is 20 ms slice for each task, with some redundant resched_tasks and
with the fix it is expected ~12ms slices (on 16 cpu system).

[1] - http://lwn.net/Articles/409680/

Signed-off-by: Venkatesh Pallipadi <venki@google.com>
---
 kernel/sched_fair.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 00ebd76..fc5ffbd 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -871,8 +871,10 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
 	if (cfs_rq->nr_running > 1) {
 		struct sched_entity *se = __pick_next_entity(cfs_rq);
 		s64 delta = curr->vruntime - se->vruntime;
+		unsigned long ideal_vruntime;
 
-		if (delta > ideal_runtime)
+		ideal_vruntime = calc_delta_fair(ideal_runtime, curr);
+		if (delta > (s64)ideal_vruntime)
 			resched_task(rq_of(cfs_rq)->curr);
 	}
 }
-- 
1.7.3.1